1. Field of the Invention
This invention relates to an image sensing apparatus and more particularly to an apparatus of the kind having a field/frame image sensing mode selecting function.
2. Description of the Related Art
It is well known that an image sensing apparatus arranged to permit telephoto shooting by means of a zoom lens, an interchangeable lens, or the like, tends to be shaken by vibrations of the hand to give a blurred picture when a long focal length of the lens is selected. This tendency becomes more salient in the event of a relatively long image sensing exposure time.
To prevent the introduction of hand vibrations to the apparatus, the conventional cameras of the kind using a silver halide film or the like have been arranged to shorten the length of exposure time with the shutter speed increased by using a high sensitivity film.
However, in the case of an electronic image sensing apparatus which uses a solid-state image sensor made of a CCD (charge-coupled device) or an MOS (metal-oxide semiconductor) or the like, a camera tube or the like, the sensitivity of the image sensor or the camera tube is not high enough to permit a sufficiently high shutter speed. In addition to that, the apparatus of this kind tends to have an under exposure because of its low degree of sensitivity not only in the case of telephoto shooting but also in the event of, for example, a low luminance object existing indoors. Therefore, the image sensing apparatus of this kind has necessitated some arrangement either to have recourse to the flash photography in the same manner as in the case of the film camera or to make up for the insufficiency of exposure by raising the gain of a video signal during a signal processing operation after image sensing at the expense of quality in terms of the S/N ratio of the signal.
Despite of the arrangement to have recourse to a flash device, however, the conventional electronic image sensing apparatus has often had an under-exposure. Besides, in case of telephoto shooting or the like where the effect of the flash photography is not adequately attainable, there has been provided no appropriate means for making up for the deficiency of exposure.